This invention relates generally to heat exchange assemblies for heating or cooling a process fluid, and more particularly to an assembly adapted to raise or lower the temperature of de-ionized process water without in any way affecting its ultra-pure condition.
The principal method used in making semiconductor devices for inclusion in monolithic or hybrid integrated circuits is planar technology whereby components such as transistors that are fabricated by the process extend below the surface of one plane of a silicon substrate or a substrate of glass or ceramic material. Fabrication of an integrated circuit involves various wet chemistry steps, many of which require critical cleaning operations in which the surfaces are rinsed with water. Because of the high sensitivity to contamination of wet chemistry processing in microelectronics, these rinsing phases must make use of water of extreme purity.
Though the concern of the present invention is primarily with heating ultra-pure de-ionized water while preserving its purity so that the heated water may be used to rinse the surfaces of microelectronic substrates, it is to be understood that a heat exchange assembly in accordance with the invention is by no means limited to this application and may be employed wherever the need exists in chemical and industrial processing for ultra-pure water at elevated or reduced temperatures. Also, the assembly, because it isolates the liquid being heated or cooled from the heating or cooling source by a chemically-inert layer, may be used in conjunction with corrosive liquids.
In the early years of microcircuit manufacturing, rinsing was performed with ultra-pure water at non-elevated temperatures, the temperature of the water being that prevailing at the point-of-use. The current practice, however, is to rinse with very hot demineralized water. Water cannot be purified to its ultimate state after being heated, the reason for this being that demineralizing resins are generally unsuitable for use at elevated temperatures. It is mandatory, therefore, that the heating apparatus perform its function at the final point-of-use, and that it assume full responsibility for maintaining the process water in its ultra-pure, demineralized condition.
In order to provide heated ultra-pure water at a point-of-use, it has heretofore been the practice to flow the water through a heat exchanger having smooth, crevice-free, internal metal structures. And since many metals conventionally used in heat exchangers, such as copper, tend to leach metallic ions from their surface which contaminate water, use is made in known types of heat exchangers for heating ultra-pure water of a non-leaching, non-corrosive metal such as passivated stainless steel. Other metals having minimal contaminating properties are block tin and tin-lined metals.
More recently, it has been found that certain plastics, especially those in the fluorocarbon family, are highly suitable for use in ultra-pure water heaters. Such inert plastics, when in contact with water, do not measurably degrade the quality of the water and are not subject to leaching of objectionable contaminants.
It has heretofore been proposed to provide in a heat-exchanger for heating ultra-pure water, tubes of polytetrafluoroethylene material (PTFE) for conducting steam or other hot fluid, the tubes being immersed in a bath of ultra-pure water in heat-exchange relationship therewith.
PTFE or "Teflon," as it is marketed by the duPont company, is highly resistant chemically within the limits of its thermal stability; for it is only affected by molten alkali metals and elemental fluorine at high pressures. Hence PTFE will not react with heated water, nor is it affected by temperatures up to 500.degree. F., this being considerably higher than the temperature involved in heating ultra-pure water.
But because Teflon is a relatively poor heat conductor, should relatively thick-walled Teflon tubes be used in a heat exchange assembly, thermal transfer will be poor and difficulties will be experienced in effecting temperature control. On the other hand, a very thin Teflon tube, though providing much better heat transfer, is not only incapable of operating under pressure conditions, but its plastic walls often incorporate pores or pinholes. This gives rise to minute leakage paths resulting in contamination.
One possible solution to the problem of employing Teflon in a heat exchanger for ultra-pure water is to use metal structures coated with Teflon, so that while only the Teflon makes contact with the ultra-pure water, the underlying metal structure is capable of withstanding the operating pressures involved. Even though a very thin PTFE coating will not act as a significant thermal barrier and therefore not adversely affect the heat transfer characteristic of the unit, such thin coatings almost invariably suffer from some degree of porosity and will therefore permit ions to leach from the metal structure.
Moreover, with Teflon coating techniques, there is also an adhesion problem, so that it is usually necessary to first roughen or grit blast the substrate and then apply an appropriate primer thereto before applying the coating. Even then adhesion is not assured; for the wide difference in thermal expansion characteristics between metals and plastics may result in flaking of the coating and separation of the bond. This is particularly bothersome in the context of the significant temperature cycling operations inherent in many process applications.